I suggest you ...

YouTube Syncing in Text Format

In addition to the interactive interface for syncing videos, it would be nice to be able to edit the syncing in text format as well. This would be more of a power user feature that would let users tweak the syncing by hand. What I really want is SRT format support (link to that feature request below), but if that is too hard, just exposing your native syncing format for users to edit by hand would allow for external tools to do the SRT. I figure this would be an easy feature from your perspective and lean on the community to build the SRT support :)

I actually just found the Readlang API (https://github.com/SteveRidout/readlang-api), and it looks like it's already possible to set the the audioMap on the book. Perhaps just document that API for this feature request ?